Currently we use Jira Service Desk in version 4.13.3. We updated on this version shortly.
I created a new service desk project by shared configuration of another project.
The new projct has set English as the project default.
I added German as a language, the status is up to date and it is available to customers.

Though, I can not change the default language to German.
It should work this way as it is described here: https://confluence.atlassian.com/servicedeskserver0413/providing-help-in-multiple-languages-1027136636.html
But somehow it doesn't.
Does anybody know if this is a bug, if there is a workaround or if any other configurations have to be settled first?